Rhythmic oscillation is a widespread phenomenon in biology. Repetitive cycling over characteristic time intervals controls the beating of the heart, sleep cycles and hormonal release. In the heart this intrinsic rhythm, known as pacemaking, drives the pulsatile circulation of extracellular fluids that enables larger multicellular organisms to exist. Pacemaking involves several ionic currents in individual excitable cells but the exact mechanism generating this lifelong oscillation in the membrane potential is still not known. Multiple signaling pathways modulate the basic cycle length of the cardiac pacemaker. There are interactions between different cell types within the heart and mechanical, metabolic, neural, endocrine and paracrine inputs are integrated in the final response. The resultant capacity for a dynamic range of heart rate (HR) responses is vital to the organism's survival in a changing environment. This complexity has, however, made it difficult to study the molecular biology of cardiac pacemaking in an integrated way in the whole organism. This proposal details experiments designed to clone the gene responsible for a major defect in the regulation of the HR in zebrafish and thus gain an entry point for the genetic dissection of cardiac pacemaking. The zebrafish mutant slo mo (smo) displays heritable intrinsic bradycardia but exhibits no other developmental defect. It is anticipated that the cloning of this gene will offer significant insight into the establishment and regulation of cardiac pacemaking activity. The proposal has 3 specific aims; 1) to clone positionally the mutated gene responsible for the smo phenotype, 2) to investigate the function of the smo gene by characterizing its mRNA/protein expression and the effects of its inactivation using morpholinos and 3) to identify other genes in the smo pathway using expression profiling. A long-term goal of this project is to explore which genes are important in cardiac pacemaking and in the integrated physiology of HR regulation. Heart rhythm is important in the generation and maintenance of a normal functioning cardiovascular system. The cloning of genes important for normal heart rhythm and its regulation will help to improve our understanding of how perturbations in these pathways may disrupt cardiac form and function in syndromes such as congenital heart disease and heart failure.